1. Field of the Invention
The present invention relates to a radio communication device that down-converts a received signal into a baseband signal, and particularly to a radio communication device that down-converts a digital signal resulting from AD conversion at a predetermined sampling frequency into a baseband signal.
More specifically, the present invention relates to a radio communication device in which the circuit configuration of a frequency converter is simplified by utilizing a fact that a frequency ratio between a sampling frequency at which an analog signal in an IF band is subjected to AD conversion and a frequency when frequency conversion of the IF signal into a baseband signal is performed has a specific relation, and particularly to a radio communication device in which the circuit configuration of a frequency converter is simplified and an amount of operation in a stage succeeding the frequency converter is reduced.
2. Description of the Related Art
Radio communications play a wide variety of roles ranging from high-capacity basic trunk lines of terrestrial broadcasting, terrestrial microwave communications, satellite communications, satellite broadcasting and the like to access lines of mobile communications and the like. Digital radio communications that communicate digital data by radio, such as digital broadcasting, wireless LANs (Local Area Networks) and the like, have recently been a trend of the times.
In a digital radio communication, the encoding of an information source and a communication channel and the digital modulation of a transmission signal are performed on a sender side, and as opposed to the sender side, digital demodulation and the decoding of the information source and the communication channel are performed on a receiver side. The communication technology of a digital system can increase communication speed and capacity, and enhance resistances to noise, interference, and distortion, so that high quality can be achieved.
FIG. 6 shows an example of configuration of a receiver for digital radio communication. First, an RF signal received by an antenna is amplified by an LNA (Low Noise Amplifier), and once down-converted from an RF band to a predetermined IF (Intermediate Frequency) band. Next, the IF signal is amplified by a VGA (Variable Gain Amplifier). The variable gain of the VGA is specified by a baseband circuit in a subsequent stage, and input to the VGA after being subjected to analog conversion by a DA converter (DAC). Next, an AD converter (ADC) converts the IF signal into a digital signal by an AD conversion at a predetermined sampling frequency. Further, the digital signal is down-converted into complex baseband signals by digital down-converters, subjected to waveform shaping by LPFs (Low Pass Filters), and down-sampled by down-sampling circuits. Thereafter the baseband circuit formed by a digital circuit performs baseband processing.
It is known in the art that the configuration of a frequency converter can be simplified when a relation such that a ratio of the sampling frequency at the time of the AD conversion to the frequency for subjecting the digital signal after the AD conversion to the frequency conversion is 4 to 1 or 4 to 3 holds (see for example, Coauthored by Jan-Erik Eklund and Ragnar Arvidsson “A Multiple Sampling, Single A/D Conversion Technique for I/Q Demodulation in CMOS” (IEEE Journal of Solid-State Circuits, Vol. 31, No. 12, December 1996)).
In the receiver shown in FIG. 6, a digital signal obtained by performing frequency conversion of an RF signal into an IF band in a first frequency converter (frequency f1) and thereafter sampling the IF signal by AD conversion at a sampling frequency Fs is subjected to frequency conversion in a second frequency converter (frequency f2) to be converted into complex baseband signals. The complex baseband signals are then subjected to waveform shaping in the LPFs.
The second frequency conversion is as shown in the following Equations (1) and (2). In this case, time is treated as discrete times Δt×n (n=0, 1, 2, . . . , ∞), Δt=1/Fs, and f2/Fs=¼. Cosine and sine operations in this case are performed in units of 90 degrees. Results of the cosine and sine operations therefore assume three values 0, 1, and −1. That is, multiplication of cosine and sine waveforms is not necessary, and cosines and sines can be treated as frequency conversion coefficients as shown in FIG. 7. Thus, zero and a sign inversion are used for the multiplication of the received signal r(t) after the AD conversion by a cosine and a sine, so that an amount of operation is greatly reduced (see Japanese Patent Laid-Open No. 2002-76975, paragraphs 0015 and 0016, for example).
                              I          ⁡                      (            t            )                          =                                            r              ⁡                              (                t                )                                      ·                          cos              ⁡                              (                                                      2                    ·                    π                    ·                    f                                    ⁢                                                                          ⁢                                      2                    ·                    Δ                                    ⁢                                                                          ⁢                                      t                    ·                    n                                                  )                                              =                                    r              ⁡                              (                t                )                                      ·                          cos              ⁡                              (                                  2                  ·                  π                  ·                                      n                    4                                                  )                                                                        (        1        )                                          Q          ⁡                      (            t            )                          =                                            -                              r                ⁡                                  (                  t                  )                                                      ·                          sin              ⁡                              (                                                      2                    ·                    π                    ·                    f                                    ⁢                                                                          ⁢                                      2                    ·                    Δ                                    ⁢                                                                          ⁢                                      t                    ·                    n                                                  )                                              =                                    -                              r                ⁡                                  (                  t                  )                                                      ·                          sin              ⁡                              (                                  2                  ·                  π                  ·                                      n                    4                                                  )                                                                        (        2        )            
FIG. 8 shows a configuration of a receiver whose circuit configuration is simplified by representing the multiplication of the received signal r(t) resulting from the AD conversion by the cosine and the sine by zero and a sign inversion. The receiver is the same as in FIG. 6 in that an RF signal received by an antenna is amplified by an LNA and then converted into an IF band by a frequency conversion by a first frequency converter (frequency f1), and next the IF signal is amplified by a VGA and converted into a digital signal by an AD conversion at a sampling frequency F, by an AD converter. In a second frequency converter, the multiplication of the received signal r(t) after the AD conversion by the cosine and the sine with the frequency f2 is replaced with the multiplication of the received signal r(t) after the AD conversion by the frequency conversion coefficients shown in FIG. 7.
FIGS. 9A to 9D show states of signals at respective points in the receiver shown in FIG. 8.
r(t) denotes a received signal resulting from AD conversion at the sampling frequency Fs. I0(t) and Q0(t) denote signals down-converted at the frequency f2 by the frequency conversion coefficients shown in FIG. 7. FIG. 9B shows that the signals I0(t) and Q0(t) include zero in every other sample. This is due to zero alternately included in the frequency conversion coefficients.
I1(t) and Q1(t) denote signals obtained by performing LPF processing on the frequency-converted signals I0(t) and Q0(t). FIG. 10 shows an example of configuration of a digital LPF.
I2(t) and Q2(t) denote results obtained by performing discrete reduction processing on the signals I1(t) and Q1(t) after the LPF processing by down-sampling circuits. In the example shown in FIG. 9D, discrete reduction is performed in every other sample, and thereby a rate is halved.